Flaky e2e tests in Travis

Recently tests failures reported quite often on `make test-ui-e2e` step of Travis build jobs . This snippet extracted from [build job 452200226](https://api.travis-ci.org/v3/job/452200226/log.txt) illustrates details a bit better .

```
**************************************************
* Failures *
**************************************************

1) Transactions should contain transactions
- Expected 0 to be greater than 0.

2) Transactions should show transaction detail modal
- Failed: Index out of bound. Trying to access element at index: 0, but there are only 0 elements that match locator By(css selector, .-transaction)

3) Transactions should hide transaction detail modal
- Failed: Wait timed out after 5001ms

Executed 40 of 40 specs (3 FAILED) in 47 secs.
[03:29:55] I/launcher - 0 instance(s) of WebDriver still running
[03:29:55] I/launcher - chrome #01 failed 3 test(s)
[03:29:55] I/launcher - overall: 3 failed spec(s)
[03:29:55] E/launcher - Process exited with error code 1
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! desktopwallet@0.0.0 e2e-choose-config: `ng e2e --proxy-config $E2E_PROXY_CONFIG --delete-output-path false`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the desktopwallet@0.0.0 e2e-choose-config script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! /home/travis/.npm/_logs/2018-11-08T03_29_55_218Z-debug.log
+RESULT=1
+echo 'shutting down skycoin node'
shutting down skycoin node
+kill -s SIGINT 30453
+wait 30453
[2018-11-08T03:29:55Z] INFO [main]: Shutting down...
[2018-11-08T03:29:55Z] INFO [main]: Closing web interface
[2018-11-08T03:29:55Z] INFO [api]: Shutting down web interface
[2018-11-08T03:29:55Z] INFO [api]: Web interface closed
[2018-11-08T03:29:55Z] ERROR [main]: accept tcp 127.0.0.1:1024: use of closed network connection
[2018-11-08T03:29:55Z] INFO [api]: Web interface shut down
[2018-11-08T03:29:55Z] INFO [main]: Closing daemon
[2018-11-08T03:29:55Z] INFO [daemon]: Stopping the daemon run loop
[2018-11-08T03:29:55Z] INFO [daemon]: Shutting down Pool
[2018-11-08T03:29:55Z] INFO [gnet]: ConnectionPool.Shutdown called
[2018-11-08T03:29:55Z] INFO [gnet]: ConnectionPool.Shutdown closed pool.quit
[2018-11-08T03:29:55Z] INFO [gnet]: ConnectionPool.Shutdown waiting for strandDone
[2018-11-08T03:29:55Z] INFO [gnet]: ConnectionPool.Shutdown closing the listener
[2018-11-08T03:29:55Z] INFO [gnet]: ConnectionPool.Shutdown disconnecting all connections
[2018-11-08T03:29:55Z] INFO [gnet]: ConnectionPool.Shutdown waiting for done
[2018-11-08T03:29:55Z] INFO [daemon]: Shutting down Gateway
[2018-11-08T03:29:55Z] INFO [daemon]: Shutting down Pex
[2018-11-08T03:29:55Z] INFO [pex]: Shutting down pex
[2018-11-08T03:29:55Z] INFO [pex]: Save peerlist
[2018-11-08T03:29:55Z] INFO [pex]: Pex.Run stopped
[2018-11-08T03:29:55Z] INFO [pex]: Pex shutdown
[2018-11-08T03:29:55Z] INFO [daemon]: Daemon closed
[2018-11-08T03:29:55Z] INFO [daemon]: Daemon shutdown complete
[2018-11-08T03:29:55Z] INFO [main]: Waiting for goroutines to finish
[2018-11-08T03:29:55Z] INFO [main]: Closing database
[2018-11-08T03:29:55Z] INFO [main]: Goodbye
+rm skycoin-integration
+rm /tmp/e2e-proxy.config.FhQzyq.js
+[[ 1 -ne 0 ]]
+exit 1
make: *** [test-ui-e2e] Error 1

travis_time:end:229c33c0:start=1541647715908676009,finish=1541647795239705195,duration=79331029186

The command "make test-ui-e2e" exited with 2.
```