.travis.yml: install protobuf 2.5.0 from source

The travis-ci environment is based on Ubuntu 12.04 LTS, which has an
older protobuf version (2.4.1). We need to depend on features that are
only available in 2.5.0 (and, later, 2.6.0), so instead of satisfying
the protobuf build dependencies from the Ubuntu repository, download and
install our own copy of protobuf.

The travis-ci build from this commit should succeed, since we are
compatible with protobuf 2.5.0.

Based on a patch from Ilya Lipnitskiy.

.travis.yml

 language: c
 language: cpp
 
+env:
+    - PROTOBUF_VERSION=2.5.0
+
 compiler:
     - gcc
     - clang
@@ -9,8 +12,11 @@ before_install:
     - sudo apt-get update -qq
 
 install:
-    - sudo apt-get -q install protobuf-compiler libprotobuf-dev libprotoc-dev valgrind lcov
+    - sudo apt-get -q install valgrind lcov
     - 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 )
 
 script:
     - ./autogen.sh