Need CI testing on Mono (OSX)

[fsgit]

A CI build on Mac OSX using Mono 3.4.0 has been set up

The main type provider builds but the build is failing when building the tests, see https://travis-ci.org/fsprojects/ExcelProvider/builds/35113927 for example. This may well be a path problem "" instead of "/" but it isn't clear.

The steps to reproduce are in .travis.yml (basically just run build.sh)

[JohnDoeKyrgyz]

I added a better error message when the type provider cannot open an excel file on revision 15840ac. This should give us some more information to figure out if the file path is the issue when the next build goes through.

[JohnDoeKyrgyz]

The issue is not an IOException when opening the Excel file as was originally guessed. Unfortunately I don't have access to a mac, so my next step is to put some try with and printf statements in and commit to try and figure out where the error is occurring. Would anyone with access to a mac like to try and debug the type provider before I try this?