travis: Update travis to specify which unix variant to build.

.travis.yml

@@ -73,7 +73,7 @@ jobs:
         - make ${MAKEOPTS} -C mpy-cross
         - make ${MAKEOPTS} -C ports/unix submodules
         - make ${MAKEOPTS} -C ports/unix deplibs
-        - make ${MAKEOPTS} -C ports/unix coverage
+        - make ${MAKEOPTS} -C ports/unix VARIANT=coverage
         # run the main test suite
         - (cd tests && MICROPY_CPYTHON3=python3 MICROPY_MICROPYTHON=../ports/unix/micropython_coverage ./run-tests)
         - (cd tests && MICROPY_CPYTHON3=python3 MICROPY_MICROPYTHON=../ports/unix/micropython_coverage ./run-tests -d thread)
@@ -157,7 +157,7 @@ jobs:
         - make ${MAKEOPTS} -C mpy-cross PYTHON=python2
         - make ${MAKEOPTS} -C ports/unix submodules
         - make ${MAKEOPTS} -C ports/unix PYTHON=python2 deplibs
-        - make ${MAKEOPTS} -C ports/unix PYTHON=python2 nanbox
+        - make ${MAKEOPTS} -C ports/unix PYTHON=python2 VARIANT=nanbox
         - (cd tests && MICROPY_CPYTHON3=python3 MICROPY_MICROPYTHON=../ports/unix/micropython_nanbox ./run-tests)
 
     # unix stackless