Travis: add a strict build

Clang on Linux seems to catch things that we might miss otherwise.
Also, throw in 'no-deprecated' to make sure we test that as well.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1839)

.travis.yml

@@ -35,6 +35,9 @@ env:
 
 matrix:
     include:
+        - os: linux
+          compiler: clang-3.6
+          env: CONFIG_OPTS="--strict-warnings no-deprecated" BUILDONLY="yes"
         - os: linux
           compiler: gcc
           env: CONFIG_OPTS="--debug --coverage no-asm enable-rc5 enable-md2 enable-ec_nistp_64_gcc_128 enable-ssl3 enable-ssl3-method enable-nextprotoneg enable-weak-ssl-ciphers" COVERALLS="yes"