.travis.yml: run 'ldconfig' after installing protobuf

This should fix the following build failure.

    ./protoc-c/protoc-c: error while loading shared libraries: libprotobuf.so.8: cannot open shared object file: No such file or directory
    make: *** [t/test.pb-c.c] Error 127

.travis.yml

@@ -17,7 +17,7 @@ install:
     - sudo pip install cpp-coveralls
     - wget https://protobuf.googlecode.com/svn/rc/protobuf-$PROTOBUF_VERSION.tar.gz
     - tar xf protobuf-$PROTOBUF_VERSION.tar.gz
-    - ( cd protobuf-$PROTOBUF_VERSION && ./configure && make -j2 && sudo make install )
+    - ( cd protobuf-$PROTOBUF_VERSION && ./configure && make -j2 && sudo make install && sudo ldconfig )
 
 script:
     - ./autogen.sh