Remove clang-7 compiler pin for vulkan

[areusch]

• Breaks build with new 18.04 ci-gpu docker container.

[  1%] Building CXX object CMakeFiles/tvm_runtime_objs.dir/src/runtime/builtin_fp16.cc.o
/bin/sh: 1: clang-7: not found
make[2]: *** [CMakeFiles/tvm_runtime_objs.dir/src/runtime/builtin_fp16.cc.o] Error 127
CMakeFiles/tvm_runtime_objs.dir/build.make:62: recipe for target 'CMakeFiles/tvm_runtime_objs.dir/src/runtime/builtin_fp16.cc.o' failed

@tkonolige @tqchen

[tqchen]

Perhaps we want to ping to a different version of clang later, it is good to get a matrix of compiler setup coverage