Drop support for XCode 7

lmdbxx requires support for thread_local which is not available on that clang version.

.travis.yml

@@ -7,9 +7,6 @@ notifications:
 
 matrix:
     include:
-        - os: osx
-          osx_image: xcode7
-          compiler: clang
         - os: osx
           osx_image: xcode8
           compiler: clang

README.md

@@ -59,7 +59,7 @@ sudo apt-get update
 sudo apt-get install qt58base qt58tools cmake liblmdb-dev
 ```
 
-##### OSX (Xcode 7 or later)
+##### OSX (Xcode 8 or later)
 
 ```bash
 brew update