Cast MUMPS data int indices to dolfin::la_index. Caused failure of compilation with 64-bit indices in PETSc.
Merge branch 'martinal/topic-update-changelog-for-15'

Conflicts:
	ChangeLog
Merge branch 'jan/fix-mumpslusolver-int' into next
Check that GCC version is at least 4.6.

4.6 is reported to work, and 4.4 is know to fail. Not sure about 4.5, but it's pretty old anyway.
Merge branch 'garth/fix-issue-408-check-gcc-version' into next