Flapping tests

Nobody has claimed this yet.

Recently travis tests have failed every once in a while due to network failures. Ticketing here so other devs have some visibility that this is happening and to log any ideas for mitigating.

The most common error appears to be when running the android compile test (https://github.com/mapbox/mason/blob/master/test/c_build_android.sh). This test usually succeeds and outputs:

```
* Downloading binary package osx-x86_64/android-ndk/arm-9-r13b.tar.gz...
* Installed binary package at /Users/travis/build/mapbox/mason/mason_packages/osx-x86_64/android-ndk/arm-9-r13b
* Downloading http://nongnu.askapache.com/freetype/freetype-2.5.5.tar.bz2...
* Building for Platform 'android/arm-v7-9'...
FreeType build system -- automatic system detection
```

But every once in a while it fails because the download of `arm-9-r13b` either fails or times out and then the build falls back to trying to build `arm-9-r13b` from source. The `arm-9-r13b.tar.gz` looks to be 125 MB.

The source build then fails with:

```
The MASON_PKGCONFIG_FILE variable not found in script.sh. Please either provide this variable or override the mason_cflags function hook
```

which looks like outfall from https://github.com/mapbox/mason/pull/356.

/cc @mapsam @kkaefer

Start with test/c_build_android.sh and reproduce the Android compile test in Travis, focusing on the arm-9-r13b download and source-build fallback. Read the failure context from mapbox/mason#356. The issue does not define a specific mitigation or acceptance criteria, so completion would require establishing a reproducible fix and confirming reliable CI runs.

Written by the indexing model from the issue text.