Update .drone.jsonnet; <thread> crashes w/ g++ 4.8 on 14.04

Author: pdimov

.drone.jsonnet

@@ -98,17 +98,17 @@ local windows_pipeline(name, image, environment, arch = "amd64") =
 
 [
     linux_pipeline(
-        "Linux 14.04 GCC 4.8*",
-        "cppalliance/droneubuntu1404:1",
-        { TOOLSET: 'gcc', COMPILER: 'g++', CXXSTD: '11' },
+        "Linux 16.04 GCC 4.8",
+        "cppalliance/droneubuntu1604:1",
+        { TOOLSET: 'gcc', COMPILER: 'g++-4.8', CXXSTD: '11' },
+        "g++-4.8",
     ),
 
     linux_pipeline(
-        "Linux 14.04 GCC 4.9",
-        "cppalliance/droneubuntu1404:1",
+        "Linux 16.04 GCC 4.9",
+        "cppalliance/droneubuntu1604:1",
         { TOOLSET: 'gcc', COMPILER: 'g++-4.9', CXXSTD: '11' },
         "g++-4.9",
-        [ "ppa:ubuntu-toolchain-r/test" ],
     ),
 
     linux_pipeline(