Make closeQuietly() Java < 1.7 compatible

This allows the mockwebserver project to be built with Java 1.5 and 1.6
compilers (e.g. when working with Android).
ServerSocket only implements Closeable since Java 1.7 and thus does not
resolve when compiling as 1.5 or 1.6. This is the same as is already done
for closeQuietly(Socket).