remove "Beta" qualifier from High Sierra

[Paul Hargrove]

Apple is due to release Mac OS X 10.13 "High Sierra" on Monday, September 25, 2017.

Based on my successful testing of prior Betas from Apple, and of the current "Golden Master" (GM), we have every reason to expect the final release to work as well. FWIW the corresponding Xcode 9 release is already final and has been tested successfully.

I should update our test system from the GM to the final release on Monday and re-test.
Then I should replace "10.13beta" in README.md with just "10.13".

[Paul Hargrove]

README.md: updates/fixes to list of supported compilers

• Update the cross-reference to local-gcc.md
• Remove "beta" from refrence to Mac OS X 10.13
• Don't use square bracket in .md!
• Line break before conduit lists

Resolves issue #48 (removing beta label from High Sierra)

→ <<cset c76a226ac764>>