travis: Build urandom native module in coverage job.

Author: dpgeorge

.travis.yml

@@ -87,6 +87,7 @@ jobs:
         - make -C examples/natmod/btree ARCH=x64
         - make -C examples/natmod/framebuf ARCH=x64
         - make -C examples/natmod/uheapq ARCH=x64
+        - make -C examples/natmod/urandom ARCH=x64
         - make -C examples/natmod/ure ARCH=x64
         - make -C examples/natmod/uzlib ARCH=x64
         # test importing .mpy generated by mpy_ld.py