Ignore Gemfile.lock
Bump mini_portile dependency

Lot of things have changed in that area, so we better use latest
version.
Silence Hoe license default warning
Conditionally define cross-compilation tasks

Only define them if a suitable cross-compiler is found.

Use `Rake::ExtensionCompiler.mingw_host` to determine if a compiler
is available (this will raise an exception is none is found).
Update Travis-CI information

- Drop Rubinius builds (will be add later)
- Exclude non-portile version failures due VM issues
- No longer email me on continous failures
- Remove Ruby 1.9.2 and replace with 2.0.0 instead