enable continuous integration via craigcitro/r-travis

- ignore .Rhistory file
- mute notification mails
- don't install assertthat
- test all branches except those whose name is ending with -expt

.Rbuildignore

@@ -4,3 +4,4 @@ doc
 man-roxygen
 ^.*\.Rproj$
 ^\.Rproj\.user$
+^\.travis\.yml$

.gitignore

@@ -1,2 +1,3 @@
 *.so
 .Rproj.user
+/.Rhistory

.travis.yml

@@ -0,0 +1,24 @@
+# Sample .travis.yml for R projects.
+#
+# See README.md for instructions.
+
+language: c
+# To build on OSX, switch the previous line to
+# language: objective-c
+script: ./travis-tool.sh run_tests
+before_install:
+  - curl -OL http://raw.github.com/craigcitro/r-travis/master/scripts/travis-tool.sh
+  - chmod 755 ./travis-tool.sh
+  - ./travis-tool.sh bootstrap
+install:
+  - ./travis-tool.sh install_deps
+# Configuration options below this line are suggested options.
+notifications:
+  email:
+    on_success: change
+    on_failure: change
+# One can also restrict the branches to test, see
+#   http://about.travis-ci.org/docs/user/build-configuration/#Specify-branches-to-build
+branches:
+  except:
+    - /-expt$/