Go 1.15 introduced a new `go vet` warning
(https://golang.org/doc/go1.15#vet) for conversions of the form
`string(x)` where `x` is an integer type other than `rune` or `byte`.
This warning is enabled by default when running `go test`. As a
consequence, running `go test github.com/golang/protobuf/proto`
results in a build failure prior to this commit.

While this module is deprecated, it is still important to ensure
that it continues to work with higher versions of Go.
Update travis.yml to test up to Go1.15.

Negative nanosecond should not have negative sign after decimal point.
Add check for max and min seconds.

Fixes #1219.

Deprecate the ptypes package since all the equivalent functionality
is now directly generated with the well-known types themselves.

Use protodesc.ToFileDescriptorProto to retrieve the raw descriptors
for legacy support instead of the undocumented ProtoLegacyRawDesc method
that we expect v2 to provide.

This change will cause the legacy proto package to incur a dependency
on the descriptorpb package.

Travis-CI is dead. GitHub actions is the new hotness.
The minimally supported version is Go1.11 to make use of modules.