Add OS X to travis; fix tests to pass on OS X

.travis.yml

 language: cpp
+os:
+  - linux
+  - osx
 script: ./autogen.sh && ./configure && make distcheck
 notifications:
   email: false

src/google/protobuf/compiler/command_line_interface_unittest.cc

@@ -991,7 +991,7 @@ TEST_F(CommandLineInterfaceTest, WriteDependencyManifestFile) {
     "  optional Foo foo = 1;\n"
     "}\n");
 
-  string current_working_directory = get_current_dir_name();
+  string current_working_directory = getcwd(NULL, 0);
   SwitchToTempDirectory();
 
   Run("protocol_compiler --dependency_out=manifest --test_out=. "