Only build libopenlibm.a

Running the tests doesn't make sense, as we're buildin for Mini-OS.
Travis: test we can build mirage-xen
Update to minios-v0.5 and openlibm-v0.4.1-tal1

This allows us to build Mini-OS and Openlibm with -nostdinc.
Merge pull request #11 from talex5/minios-0.5

Update to minios-v0.5 and openlibm-v0.4.1-tal1