Re-enable H/2 on the default HTTP client for Go 1.15+

[dcarney-stripe]

Go 1.15 fixes a long-standing bug in the HTTP/2 client that led to HTTP/2 being disabled by default in stripe-go as part of #903.

This PR modifies the default HTTP/2 client for those users running Go 1.15 or above to remove the restriction against using HTTP/2. It accomplishes this via the use of a +build tag (aka conditional compliation) of the new source file stripego115.go.

The bug is scheduled to be backported to 1.13.16 and 1.14.8, but even once those backports are complete, we can only conditionally compile against a major release of Go, per https://golang.org/cmd/go/#hdr-Build_constraints:

• a term for each Go major release, through the current version:
  “go1.1” from Go version 1.1 onward, “go1.12" from Go 1.12, and so on.

[CLAassistant]

All committers have signed the CLA.

[brandur-stripe]

Thanks @dcarney-stripe! This LGTM.

We can re-evaluate a potentially broader application of an HTTP/2 client in a few months depending on how those backports are coming.