Document how to encrypt travis secrets

[pavolloffay]

Resolves #740

cc) @yurishkuro

[codecov]

Codecov Report

Merging #744 into master will increase coverage by 0.17%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #744      +/-   ##
============================================
+ Coverage     88.92%   89.10%   +0.17%     
- Complexity      602      603       +1     
============================================
  Files            73       73              
  Lines          2257     2257              
  Branches        294      294              
============================================
+ Hits           2007     2011       +4     
+ Misses          159      155       -4     
  Partials         91       91              

Impacted Files	Coverage Δ	Complexity Δ
...ing/internal/samplers/RemoteControlledSampler.java	91.25% <0.00%> (-1.25%)	16.00% <0.00%> (-1.00%)
...gertracing/internal/reporters/LoggingReporter.java	90.90% <0.00%> (+9.09%)	5.00% <0.00%> (+1.00%)
...rtracing/internal/reporters/CompositeReporter.java	100.00% <0.00%> (+28.57%)	7.00% <0.00%> (+1.00%)

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[yurishkuro]

thanks

[yurishkuro]

Probably not related to this PR, but a flaky test related to 0-padding:

io.jaegertracing.zipkin.internal.V2SpanConverterTest > testConvertSpanWith128BitTraceId FAILED
    org.junit.ComparisonFailure: expected:<[]4530fcc74d879887070b...> but was:<[0]4530fcc74d879887070b...>
        at org.junit.Assert.assertEquals(Assert.java:115)
        at org.junit.Assert.assertEquals(Assert.java:144)
        at io.jaegertracing.zipkin.internal.V2SpanConverterTest.testConvertSpanWith128BitTraceId(V2SpanConverterTest.java:310)

Restarting build.