gistfile1.txt

# sudo /opt/td-agent/embedded/bin/fluent-gem list
actionmailer (4.1.9)
actionpack (4.1.9)
actionview (4.1.9)
activemodel (4.1.9)
activerecord (4.1.9)
activesupport (4.1.9)
addressable (2.3.6)
arel (5.0.1.20140414130214)
aws-sdk-v1 (1.63.0)
bigdecimal (1.2.4)
bson (1.12.0)
bson_ext (1.12.0)
builder (3.2.2)
bundler (1.9.9)
celluloid (0.15.2)
cool.io (1.3.0)
diff-lcs (1.2.5)
docker-api (1.21.4)
draper (1.4.0)
erubis (2.7.0)
excon (0.45.3)
fluent-logger (0.4.10)
fluent-mixin-config-placeholders (0.3.0)
fluent-mixin-plaintextformatter (0.2.6)
fluent-plugin-docker-metrics (0.0.2)
fluent-plugin-file-alternative (0.2.0)
fluent-plugin-forest (0.3.0)
fluent-plugin-mongo (0.7.8)
fluent-plugin-record-reformer (0.6.1)
fluent-plugin-rewrite-tag-filter (1.4.1)
fluent-plugin-s3 (0.5.7)
fluent-plugin-scribe (0.10.14)
fluent-plugin-td (0.10.26)
fluent-plugin-td-monitoring (0.2.0)
fluent-plugin-webhdfs (0.4.1)
fluentd (0.12.7)
fluentd-ui (0.3.18)
font-awesome-rails (4.3.0.0)
haml (4.0.6)
haml-rails (0.5.3)
hike (1.2.3)
hirb (0.7.3)
http_parser.rb (0.6.0)
httpclient (2.5.3.3, 2.4.0)
i18n (0.7.0)
io-console (0.4.2)
ipaddress (0.8.0)
jbuilder (2.2.6)
jquery-rails (3.1.2)
json (1.8.2, 1.8.1)
kramdown (1.5.0)
kramdown-haml (0.0.3)
ltsv (0.1.0)
mail (2.6.3)
mime-types (2.4.3)
mini_portile (0.6.0)
minitest (5.4.2, 4.7.5)
mixlib-cli (1.5.0)
mixlib-config (2.1.0)
mixlib-log (1.6.0)
mixlib-shellout (2.0.1)
mongo (1.12.0)
msgpack (0.5.11)
multi_json (1.10.1)
nokogiri (1.6.3.1)
ohai (6.20.0)
parallel (0.6.5)
psych (2.0.5)
puma (2.11.0)
rack (1.5.2)
rack-test (0.6.2)
rails (4.1.9)
railties (4.1.9)
rake (10.1.0)
rdoc (4.1.0)
request_store (1.1.0)
rubyzip (1.1.6, 0.9.9)
sass (3.2.19)
sass-rails (4.0.4)
settingslogic (2.0.9)
sigdump (0.2.2)
sprockets (2.11.3)
sprockets-rails (2.2.0)
string-scrub (0.0.5)
sucker_punch (1.0.5)
systemu (2.5.2)
td (0.11.8.2)
td-client (0.8.68)
td-logger (0.3.24)
test-unit (2.1.5.0)
thor (0.19.1)
thread_safe (0.3.5)
thrift (0.8.0)
tilt (1.4.1)
timers (1.1.0)
tzinfo (1.2.2)
tzinfo-data (1.2015.2)
uuidtools (2.1.5)
webhdfs (0.6.0)
yajl-ruby (1.2.1)

# td-agent --version
td-agent 0.12.7
# apt-cache show td-agent
Package: td-agent
Status: install ok installed
Priority: extra
Section: misc
Installed-Size: 207309
Maintainer: Treasure Data, Inc
Architecture: amd64
Version: 2.2.0-0
Description: Treasure Agent: A data collector for Treasure Data
Description-md5: 9a8c3c5185974525f7e38d7f37f5d597
License: unknown
Vendor: Omnibus <omnibus@getchef.com>
Homepage: http://treasuredata.com

warn.txt

# sudo /opt/td-agent/embedded/bin/fluent-gem list
WARN: Unresolved specs during Gem::Specification.reset:
      json (>= 1.4.3)
WARN: Clearing out unresolved specs.
Please report a bug if this causes problems.
(snip)

fwiw, I get the same issue

Find solutions at StackOverflow: https://stackoverflow.com/a/37057239/5157221!