fix JuMPArrays on 0.7

[mlubin]

The JuMPArray tests now pass on 0.7 (there are still more failing tests).

@rdeits, there was one regression in the inference tests that I didn't try to track down.

[codecov]

Codecov Report

Merging #1397 into master will decrease coverage by 0.45%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master    #1397      +/-   ##
==========================================
- Coverage   89.78%   89.33%   -0.46%     
==========================================
  Files          25       25              
  Lines        3544     3562      +18     
==========================================
  Hits         3182     3182              
- Misses        362      380      +18

Impacted Files	Coverage Δ
src/JuMPArray.jl	64.03% <14.28%> (-12.01%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cd6457b...3e41e1f. Read the comment docs.