travis: Use nproc to count CPUs

* Simpler tool provided by coreutils.

.travis.yml

@@ -31,7 +31,7 @@ addons:
 
 before_install:
     - export PATH=$HOME/.local/bin:$HOME/protobuf/bin:$PATH
-    - export MAKEFLAGS=-j$(grep processor /proc/cpuinfo | wc -l)
+    - export MAKEFLAGS=-j$(nproc)
     - $CC --version
     - $CXX --version
     - python --version