Fix deprecations and upgrade to 0.7

[codecov-io]

Codecov Report

Merging #89 into master will increase coverage by 0.11%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   64.97%   65.09%   +0.11%     
==========================================
  Files          11       11              
  Lines         551      550       -1     
==========================================
  Hits          358      358              
+ Misses        193      192       -1

Impacted Files	Coverage Δ
src/root_object.jl	28.57% <ø> (+3.57%)	:arrow_up:
src/quadratic.jl	96.29% <ø> (ø)	:arrow_up:
src/newton1d.jl	87.27% <100%> (ø)	:arrow_up:
src/roots.jl	88.13% <100%> (ø)	:arrow_up:

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[lbenet]

I'd like to comment that the tests involve version 0.6 (they pass) and nightly, which currently is above Julia 1.0 (tests fail). I suggest to adapt travis to test 0.7, 1.0 and nightly, not caring about Julia 0.6 anymore, perhaps allowing failures in 1.0 and nightly.

[lbenet]

I guess this is already done in #92

[dpsanders]

Superseded by #94 .