This commit upgrades cmake to 2.8.12 to implement proper cmake
`find_package` support using config and export file generation.
Having this support enables users to use installed cppformat
with a simple `find_package` call. Directly using a version
from a build directory is also supported.

main.cpp:
```
 #include <cppformat/format.h>
int main(int argc, char** argv)
{
  for(int i = 0; i < argc; ++i)
    fmt::print("{}: {}\n",i,argv[i]);
  return 0;
}

```

CMakeLists.txt:
```
cmake_minimum_required(VERSION 2.8.12)

project(cppformat-test)

find_package(cppformat REQUIRED)

add_executable(cppformat-test "main.cpp")
target_link_libraries(cppformat-test cppformat)

```
Configuring when cppformat is installed under `CMAKE_INSTALL_PREFIX`: `cmake <PATH_TO_TEST_SRC>`

Configuring when cppformat is installed `ELSEWHERE`: `cmake -Dcppformat_DIR=<ELSEWHERE>/lib/cmake/cppformat <PATH_TO_TEST_SRC>`

Configuring when cppformat is only built: `cmake -Dcppformat_DIR=<cppformat_BUILD_DIR> <PATH_TO_TEST_SRC>`