Use the check target to run tests

This provides for tighter integration into the autotools framework. More
details: https://www.gnu.org/software/automake/manual/html_node/Tests.html#Tests

Instead of using

    rspec

to run the full test suite, instead you use

    make check

Fixes #126.
Add an :echo

This `:echo` is much like the shell `echo`: it takes a bunch of strings
and outputs them, separated by spaces, followed by a newline.
Don't distribute version.rb

version.rb is built from version.rb.in by `make`, so it shouldn't be
distributed, but it should be installed.