Revert to psycopg2, and add instructions for no-binary install.

[j-bennet]

Revert to psycopg2 package insteaf of forcing psycopg2-binary.

For 2.7, this will install from wheels by default, and from source if --no-binary flag is used.

For 2.8, we need to figure out something else, since psycopg2 will become source only.

Related:
#849
#844

[codecov-io]

Codecov Report

Merging #859 into master will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #859      +/-   ##
==========================================
- Coverage   85.72%   85.58%   -0.14%     
==========================================
  Files          20       20              
  Lines        2290     2290              
==========================================
- Hits         1963     1960       -3     
- Misses        327      330       +3

Impacted Files	Coverage Δ
pgcli/main.py	77.56% <0%> (-0.49%)	⬇️

---

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 7ea9102...b95ac55. Read the comment docs.