Summary:
Update fmt version to 6.2.1 for better compatibility with the version used in fbcode. Among other things this fixes fbthrift build failure on Travis:

```
/home/fbthrift/thrift/lib/cpp2/async/RocketClientChannel.cpp:70:67:   required from here
/home/install/include/fmt/core.h:492:3: error: static assertion failed: don't know how to format the type, include fmt/ostream.h if it provides an operator<< that should be used
```
which is caused by trying to format an enum class without a formatter - only supported as of 6.0.

Reviewed By: stevegury, avalonalex

Differential Revision: D21860076

fbshipit-source-id: 1857ab65822956b005980b8dfff7a967508f507c