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"
```