Configure Travis to test Ruby 2.0.0 and 2.1.2

To ensure that we support Ruby versions other than 1.9.3.

I've chosen 2.1.2 as it's the latest stable release. 2.0.0 is the
previous stable release.
Workaround for change in Ruby 2.1.2 in test

Use a regular expression match in one of the spec tests rather than a
straight `eq()` comparison to workaround this bug in Ruby 2.1.x:

https://bugs.ruby-lang.org/issues/9285

Previously we were asserting that the error would be equal to:

```
"No such file or directory - non-existent.yaml"
```

In Ruby 2.1.x, the error raised is:

```
"No such file or directory @ rb_sysopen - non-existent.yaml"
```
Merge pull request #36 from gds-operations/73308952-test-more-ruby

[#73308952] Configure Travis to test Ruby 2.0.0 and 2.1.2