elasticsearch image updates

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this PR updates the elasticsearch versions available for Pelias:

• 6.8.3->6.8.5
• 7.4.0->7.4.2

this will allow users upgrading to ES6 to land on the most recent release in the 6 series and allow us to start testing against the most recent release in the 7 series.

the 5 series is also out-of-date but I didn't bother updating that as we're about to drop support.

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docker push pelias/elasticsearch:6.8.5
The push refers to repository [docker.io/pelias/elasticsearch]
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6.8.5: digest: sha256:b0f61a7787cdc187beeee59131b542dd4bca339341e7e7da31df69de74d0e623 size: 3675

docker push pelias/elasticsearch:7.4.2
The push refers to repository [docker.io/pelias/elasticsearch]
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7.4.2: digest: sha256:3ebac6ba9e4f90ecb11479f5acea87b28b5ee9321a8147e2e761bdf25d2586d0 size: 3669