Run R tests in addition to C++ tests (#429)

Modify _.travis.yml_:

* Change build image to R
* run _Rscript build_package.R_ as the last _script_ step
* Run C++ tests only once, under valgrind

closes #283

.travis.yml

@@ -1,5 +1,12 @@
 sudo: required
-language: cpp
+language: R
+latex: false
+cache: packages
+
+r_binary_packages:
+  - devtools
+  - testthat
+  - roxygen2
 
 matrix:
   include:
@@ -22,11 +29,15 @@ matrix:
       env: COMPILER=clang++-3.7
 
 before_install:
+  - cd r-package/grf
   - sudo apt-get update -qq
   - sudo apt-get install -y libopencv-dev valgrind
+
 script:
-  - cd core
+  - cd ../../core/
   - mkdir build && cd build
   - cmake -DCMAKE_CXX_COMPILER=$COMPILER .. && make
-  - cd .. && ./build/grf exclude:[characterization]
+  - cd ..
   - valgrind --leak-check=full --error-exitcode=1 ./build/grf exclude:[characterization]
+  - cd ../r-package
+  - Rscript build_package.R