fix travis?

Builds are hitting this issue rubygems/bundler#3559

Example https://travis-ci.org/schneems/get_process_mem/jobs/134455148

```
Bundler version 1.7.6
$ gem --version
2.5.1
11.90s$ bundle install --jobs=3 --retry=3
Fetching gem metadata from https://rubygems.org/.........
Resolving dependencies...
Installing sys-proctable 1.0.0
Installing rake 10.5.0
Using bundler 1.7.6
NoMethodError: undefined method `spec' for nil:NilClass
Installing power_assert 0.3.0
An error occurred while installing get_process_mem (0.2.1), and Bundler cannot
continue.
Make sure that `gem install get_process_mem -v '0.2.1'` succeeds before
```

.travis.yml

@@ -1,4 +1,8 @@
 language: ruby
+
+before_install:
+  - gem install bundler -v 1.12.5
+
 rvm:
   - 1.9.3
   - 2.0.0