build: Add libgcrypt20-dev to package list for travis to install.

Signed-off-by: Philip Tricca <philip.b.tricca@intel.com>
diff --git a/.travis.yml b/.travis.yml
index af4ccd1..62fdb8a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -23,6 +23,7 @@
     packages:
     - autoconf-archive
     - cmake
+    - libgcrypt20-dev
     - liburiparser-dev
     - realpath
     - lcov