.travis.yml: try to speed up the build a bit with "make -j2"

According to the travis-ci documentation, VMs are executed with 1.5
virtual CPU cores, so this should speed up our builds a little bit.

.travis.yml

@@ -21,9 +21,9 @@ install:
 
 script:
     - ./autogen.sh
-    - ./configure && make distcheck VERBOSE=1 && make clean
-    - ./configure --enable-valgrind-tests && make distcheck DISTCHECK_CONFIGURE_FLAGS="--enable-valgrind-tests" VERBOSE=1 && make clean
-    - if [ "$CC" = "gcc" ]; then ./configure --enable-code-coverage && make && make check; fi
+    - ./configure && make -j2 distcheck VERBOSE=1 && make clean
+    - ./configure --enable-valgrind-tests && make -j2 distcheck DISTCHECK_CONFIGURE_FLAGS="--enable-valgrind-tests" VERBOSE=1 && make clean
+    - if [ "$CC" = "gcc" ]; then ./configure --enable-code-coverage && make -j2 && make check; fi
 
 after_success:
     - if [ "$CC" = "gcc" ]; then cpp-coveralls --build-root . --exclude t/ --exclude /usr/include; fi