.travis.yml: use system protobuf packages and build using distcheck

the change that required protobuf >= 2.5.0 has been reverted, so there's
no need to build protobuf from source. use the system protobuf packages
instead.

also use distcheck to build and check the distribution. this will run
the test suite and perform various other checks. it will also perform a
VPATH build using the distribution tarball, which will catch any files
present in the git repository but inadvertently not distributed in the
tarball.

also test the build when configured with "--enable-rpc".

.travis.yml

-language: c
+language: cpp
 
 before_install:
-        - sudo apt-get update -qq
+    - sudo apt-get update -qq
 
 install:
-        # Install protobuf-2.5.0
-        - wget http://protobuf.googlecode.com/files/protobuf-2.5.0.tar.gz
-        - tar -xzvf protobuf-2.5.0.tar.gz
-        - cd protobuf-2.5.0 && ./configure
-        - make -j8
-        - sudo make install && sudo ldconfig
-        - cd ../
+    - sudo apt-get -q install protobuf-compiler libprotobuf-dev libprotoc-dev
 
 script:
-        - ./autogen.sh
-        - ./configure
-        - make -j8
-        - make check
-        - sudo make install
+    - ./autogen.sh
+    - ./configure && make distcheck && make clean
+    - ./configure --enable-rpc && make distcheck DISTCHECK_CONFIGURE_FLAGS="--enable-rpc" && make clean