Avoid O_CLOEXEC to support OCaml 4.00
Added Travis tests

Tests building first just stubs, then tracing for Unix, then tracing for
Unix and Xen.
Removed C stubs for adding timestamp on Xen

Use the monotonic time support that will soon be added to mirage-xen
instead.

This avoids a build cycle (we need the mirage-xen headers to build the C
stubs, but mirage-xen depends on mirage-profile). Another way to solve
this would be to split mirage-profile into two OPAM packages.