Travis compiles with clang too.
Dont install Boost.
Rebased and made use of CXX environment variable to detect compiler.

Conflicts:
	.travis.yml
The string g++ is also included in clang++, fixed.
fixed issue where ogs-gui would crash if the object selected in the pipeline was removed
[FEMCondition] Returning const& to vector instead of a copy of the vector.
[FEMCondition] Added initGeometricAttributes().
[FEMCondition] Removed old OGS-5 code.
Merge pull request #308 from TomFischer/FemBC

Init FEM condition geometry
Fixed issue where mesh to surface conversion would fail if points had been collapsed when creating geometry point vector; current solution allows for triangles with A=0.
Merge branch 'master' of https://github.com/ufz/ogs
Merge pull request #268 from bilke/travis-clang

Enable Clang on Travis
Merge branch 'master' of https://github.com/ufz/ogs into petsc_vector
Changes according to the more comments from Dima and Nori