Revert requiring guava 17

see https://issues.apache.org/jira/browse/JCLOUDS-534 for discussion
nope. the true flag meant swallow IOE, so this form was accidentally redundant.

http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Closeables.html#close(java.io.Closeable,%20boolean)