add support for keras 2.3

[matthew-z]

• make losses inherit keras.losses.Loss
• replace some keras.backend.tf with tf (K.tf does not exist anymore as keras is going to be synced with tf.keras and drop multi-backend)

We may consider to replace keras with tf.keras in the future.

fix #789

[matthew-z]

somehow the CI failed because it failed to install tensorflow==2.0.0

[uduse]

instead of using 2.0.0, choose one version from: 2.0.0a0, 2.0.0b0, 2.0.0b1

[matthew-z]

2.0.0 official version has been released, but the pip used in trusty is too old to install it. I will add commands to upgrade pip in the .travis.yml and have a try again.

[codecov-io]

Codecov Report

Merging #791 into master will increase coverage by 0.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #791      +/-   ##
==========================================
+ Coverage    96.3%   96.31%   +0.01%     
==========================================
  Files          83       83              
  Lines        2541     2549       +8     
==========================================
+ Hits         2447     2455       +8     
  Misses         94       94

Impacted Files	Coverage Δ
matchzoo/models/drmm.py	100% <ø> (ø)	:arrow_up:
matchzoo/layers/matching_layer.py	91.8% <100%> (+0.13%)	:arrow_up:
matchzoo/losses/rank_cross_entropy_loss.py	100% <100%> (ø)	:arrow_up:
matchzoo/losses/rank_hinge_loss.py	90.47% <100%> (+1.58%)	:arrow_up:
matchzoo/models/conv_knrm.py	97.82% <100%> (-0.05%)	:arrow_down:
matchzoo/models/drmmtks.py	100% <100%> (ø)	:arrow_up:
matchzoo/models/mvlstm.py	100% <100%> (ø)	:arrow_up:
matchzoo/models/duet.py	100% <100%> (ø)	:arrow_up:
matchzoo/layers/dynamic_pooling_layer.py	91.17% <100%> (ø)	:arrow_up:
matchzoo/models/knrm.py	100% <100%> (ø)	:arrow_up:

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[matthew-z]

OK, CI passed

[uduse]

Wait, this is not on dev branch... You are supposed to make changes on branch 2.2-dev. Could you please do so?

[uduse]

I guess after this PR is merged, we will release a 2.3 version.

[matthew-z]

OK I will rebase to dev-2.2 and open a new PR