.travis.yml: remove generated directories

* .travis.yml (before_script): remove directories generated
  previously.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47227 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

.travis.yml

@@ -40,6 +40,7 @@ install: "sudo apt-get -qq build-dep ruby1.9.1 2>/dev/null"
 before_script:
   - "uname -a"
   - "uname -r"
+  - "rm -fr .ext autom4te.cache"
   - "make -f common.mk BASERUBY=ruby srcdir=. update-config_files"
   - "autoconf"
   - "mkdir config_1st config_2nd"