[gmime] Updated Travis-CI build config to install libidn2

[Rico Tzschichholz <ricotz src gnome org>]

commit 8a763a8aac012733b80de0df8eb70ec2915c896d
Author: Jeffrey Stedfast <jestedfa microsoft com>
Date:   Sat Oct 27 12:00:25 2018 -0400

    Updated Travis-CI build config to install libidn2

 .travis.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.travis.yml b/.travis.yml
index 2c242413..80fc6d06 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -5,7 +5,7 @@ compiler:
 sudo: required
 before_install:
   - sudo apt-get -qq update
-  - sudo apt-get install -y automake gtk-doc-tools gnome-doc-utils libgpgme11 libgpgme11-dev gpgsm lcov
+  - sudo apt-get install -y automake gtk-doc-tools gnome-doc-utils libgpgme11 libgpgme11-dev gpgsm lcov 
libidn2-dev
 install:
   - pip install --user cpp-coveralls
   - gem install coveralls-lcov