travis: add libssl-dev

It is needed in integ test for hash calculation.

Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
diff --git a/.travis.yml b/.travis.yml
index c19b8c6..af4ccd1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -26,6 +26,7 @@
     - liburiparser-dev
     - realpath
     - lcov
+    - libssl-dev
   coverity_scan:
     project:
       name: "01org/tpm2-tss"