unicode, x/text: unit tests fail on Go1.7

[dsnet]

A recent change a2a4db7 causes the following tests to fail:

• golang.org/x/text/cases
• golang.org/x/text/secure/precis

This is using golang/txt@4440cd4f4c2ea31e1872e00de675a86d0c19006c.

/cc @mpvl

[mpvl]

Should be fixed now. Will check in the upgrade to Unicode 9.0.0 for x/text soon after verifying the reverse issue does not exist.

[quentinmit]

See also #11811