Verify that Mono isn't busted.

There was a bug in Mono's method overload resolution that I found last
August. The Mono team fixed it promptly, but if you don't have the fix on
your machine, it will cause the Irony parser to behave badly. This is the
underlying cause of Issue #24.

The fixed Mono isn't in a patched Ubuntu 12.10, which is used by
Travis-CI.

Here I create a unit test for this Mono bug, and modified .travis.yml to run
that test before doing anything else.
Use `-` instead of `/` on the NUnit command line.

Linux sees the slash as part of a path.

The quotes are required to prevent PowerShell from parsing it as a
parameter, which produces the wrong result.
Merge pull request #27 from JayBazuzi/monobug6541

Issue #24 - verify Mono bug 6541 is fixed