.travis.yml: fix 'language' declaration

It turns out that in YAML,

    language: c
    language: cpp

Actually means:

    language: cpp

I'm not sure if travis-ci actually supports providing a list of values
for the 'language' parameter, but let's try it anyway.

.travis.yml

-language: c
-language: cpp
+language:
+    - c
+    - cpp
 
 env:
     - PROTOBUF_VERSION=2.5.0