travis: add separate "scons" and "scons llvm" targets

travis: add separate "scons" and "scons llvm" targets

The former does not require any LLVM, while the latter uses LLVM 3.3.

This way we'll quickly catch any LLVM 3.3+ functionality that gets
introduced where it shouldn't.

Add the full list of addons for each build permutation.

v2: Keep libedit-dev, rework check target.
v3: Comment the current check target, add -j4 SCONSFLAGS
v4:
 - Remove llvm-toolchain-trusty-3.3 source (Andres)
 - Keep check target as-is (Andres)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Andres Gomez <agomez@igalia.com>