Intermittent build halt while installing Ruby gem pg_query

As of ~30 mins ago, the build has been halting:

```
Installing ethon 0.10.1
Installing pg_query 0.9.2 with native extensions

No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received

The build has been terminated
```

https://travis-ci.org/coala/docker-coala-base/builds/204459620

Normally the sequence is
```
...
Installing sass 3.4.23
Using bundler 1.13.6
Installing tzinfo 1.2.2
Installing descendants_tracker 0.0.4
Installing addressable 2.5.0
Installing parser 2.4.0.0
Installing mime-types 3.1
Installing ethon 0.10.1
Installing pg_query 0.9.2 with native extensions
Installing scss_lint 0.52.0
Installing activesupport 4.2.8
...
```
https://travis-ci.org/coala/docker-coala-base/builds/204445265

@gaocegege

Start by comparing the failing Travis build 204459620 with the successful build 204445265, focusing on installation of pg_query 0.9.2 with native extensions. Check the Travis build configuration described in the linked common-build-problems guidance. Done means the dependency installation completes reliably without the no-output timeout.

Written by the indexing model from the issue text.