.travis.yml: Drop 'clean' from test script

The 'clean' target has been included there since 1f34600 (Add travis
tests, 2016-05-27, #46).  But 'clean' has been a 'release'
prerequisite since 04848d1 (Add release build process, 2016-05-27,
#47), so the .travis.yml entry is redundant.

.travis.yml

@@ -7,4 +7,4 @@ go:
   - 1.9.x
   - 1.10.x
 script:
-  - make clean release
+  - make release