Require Julia 0.7

[anriseth]

I just want to see what femtocleaner has to say here (Ref #126 )

[codecov]

Codecov Report

Merging #127 into master will increase coverage by 1.46%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
+ Coverage   65.03%   66.49%   +1.46%     
==========================================
  Files           8        8              
  Lines         652      388     -264     
==========================================
- Hits          424      258     -166     
+ Misses        228      130      -98

Impacted Files	Coverage Δ
src/strongwolfe.jl	73.52% <ø> (-0.48%)	⬇️
src/hagerzhang.jl	48.88% <0%> (-13.4%)	⬇️
src/backtracking.jl	86.2% <100%> (+4.15%)	⬆️
src/static.jl	87.5% <100%> (+55.14%)	⬆️
src/LineSearches.jl	100% <100%> (+10.71%)	⬆️
src/initialguess.jl	87.01% <50%> (-1.09%)	⬇️
... and 3 more

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[anriseth]

There were lots of deprecation warnings from HigherPrecision because T(0) no longer defaults to convert(T, 0).
I have therefore changed all lines calling T(<number>) to convert(T, <number>). It does not look as nice, but the alternative would have to be to rely on all number types defining T(x::Number) = convert(T, x) ...