Faster builds by limiting visible CPUs

As described in this elm compiler issue:
elm/compiler#1473

.travis.yml

@@ -10,17 +10,26 @@ addons:
 env:
   - TRAVIS_NODE_VERSION="v7.2.1"
 
+cache:
+  directories:
+    - sysconfcpus
+    - protoc
+
 before_install:
   - rm -rf ~/.nvm && git clone https://github.com/creationix/nvm.git ~/.nvm && (cd ~/.nvm && git checkout `git describe --abbrev=0 --tags`) && source ~/.nvm/nvm.sh && nvm install $TRAVIS_NODE_VERSION
+  - ./install_sysconfcpus
+  - ./install_protobuf
 
 install:
-  - ./install_protobuf
   - export PATH=$PATH:$PWD/protoc/bin
   - go get -t -v ./protoc-gen-elm
   - protoc --version
   - node --version
   - npm --version
   - npm install -g elm
   - npm install -g elm-test
+  - mv $(npm config get prefix)/bin/elm-make $(npm config get prefix)/bin/elm-make-old
+  - printf '%s\n\n' '#!/bin/bash' 'echo "Running elm-make with sysconfcpus -n 2"' '$TRAVIS_BUILD_DIR/sysconfcpus/bin/sysconfcpus -n 2 elm-make-old "$@"' > $(npm config get prefix)/bin/elm-make
+  - chmod +x $(npm config get prefix)/bin/elm-make
 
 script: ./run_tests

install_sysconfcpus

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+set -ex
+
+echo "Install libsysconfcpus to compensate for slow elm builds"
+# More at: https://github.com/elm/compiler/issues/1473
+if [ ! -d sysconfcpus/bin ]; then
+    git clone https://github.com/obmarg/libsysconfcpus.git
+    cd libsysconfcpus
+    ./configure --prefix=$TRAVIS_BUILD_DIR/sysconfcpus
+    make && make install
+    cd ..
+fi