[TEST] BUILD: Make Travis clean all temporary build files before each build

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
MISC: Minor style fixes

- Fixed "copyable but non assignable classes" warnings
- Minor style realted fixes (no ctor/dtor oneliners)

Signed-off-by: Gama <misept.dieseneuf@gmail.com>