Fix travis for Linux build

[murlock]

Several issues was detected with linux build:

• travis_wait raise timeout
• --jobs=2 sometimes fails
• add flag BOOST_NO_CXX11_SCOPED_ENUMS to fix compilation with old boost filesystme
• add missing tool xmllint

[CelticMinstrel]

Looks good, except that I think the Boost flag would be better passed on the command-line rather than baked into the Sconstruct (unless it's also needed for you to build on your computer?). I'm not sure whether you'd need to pass it as a flag to scons or set it in the environment; I can see at least that scons seems to be inheriting the environment.

[CelticMinstrel]

Merged, thanks!