Wrong prices displayed on summary for formula assets

[Zapata]

If I run HERTZ.yaml (after fixing #37), I have inconsistent information displayed on the console:

$ python3 cli.py --configfile bitshares_pricefeed/examples/hertz.yaml update

+--------+------------+--------------+--------------+------------------------+------------------------+------------------------+--------------+------------------------+--------+--------+-------------------------+-------------+
| symbol | collateral |  new price   |     cer      |          mean          |         median         |       wgt. avg.        | wgt. std (#) |       blockchain       |  mssr  |  mcr   |      my last price      | last update |
+--------+------------+--------------+--------------+------------------------+------------------------+------------------------+--------------+------------------------+--------+--------+-------------------------+-------------+
| HERTZ  |    BTS     | 4.7511676181 | 0.2525694937 | 0.2104745781 (2157.36) | 0.2104745781 (2157.36) | 0.2104745781 (2157.36) |  0.00 ( 1)   | 0.2262904320 (1999.59) | 110.0% | 200.0% | -1.0000000000 (-575.12) | unknown ago |
+--------+------------+--------------+--------------+------------------------+------------------------+------------------------+--------------+------------------------+--------+--------+-------------------------+-------------+

As you can see new price = 4.7511676181 and cer = 0.2525694937, it seems inverted.

[Zapata]

This commit fixes this issue and #37.