Update .travis.yml
diff --git a/.travis.yml b/.travis.yml
index a4c5183..6b55737 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -27,7 +27,7 @@
           sources:
             - *common_sources
             - sourceline: 'deb http://apt.llvm.org/jessie/ llvm-toolchain-jessie main'
-              key_url: 'http://apt.llvm.org/llvm-snapshot.gpg.key'
+              key_url: 'https://apt.llvm.org/llvm-snapshot.gpg.key'
 
     - env: BUILD_TYPE=Debug CC_COMPILER=clang-${CLANG_VERSION} CXX_COMPILER=clang++-${CLANG_VERSION}
       addons: *clang