~ancarda/psr7-string-stream

Introduce ReadOnlyStringStream

This commit introduces a Read Only version of StringStream
Fix write() to not prepend

This commit brings write() more inline with how fwrite() works, given
that's ultimately what StreamInterface is trying to provide; a wrapper
around streams and the various file functions

It's still possible to append by writing beyond the capacity of the
string
Attempt to build on PHP 7.4 and 8.0

8.0 is going to fail at `composer install', however 7.4 might make it
all the way to the end. Once 7.4 is out, this build can be marked as
required
Fix write() to not assume appending is correct

This commit rewrites the `write()' function to check if it's at EOF
before appending. Now the function is able to prepend if at SOF, and
can also overwrite data in the middle of a stream.
Correct path to clover for Coveralls

It seems php-coveralls will read `build/logs/clover.xml' by default
Use travis-ci.com instead of travis-ci.org