promote(rational,float) => float again [#3378]

Revert promotion behavior of rationals with floating-point values
to behavior prior to 6125749a18 and 24dbefc380, i.e. floating-point
wins. Also includes a couple of promotion tweaks for MathConsts.

See discussion at

https://groups.google.com/forum/?fromgroups#!searchin/julia-dev/rational/julia-dev/2JcZdFKisis/Ag9rBJrrqQQJ
Merge branch 'master' of github.com:JuliaLang/julia