Fix installation of elasticsearch==6.3.1
Fix installation of elasticsearch==6.3.1

**Purpose**

Currently, elasticsearch 5.4.0 gets installed, even though
pip thinks that 6.3.1 (the correct version) is installed.

```python
>>> import elasticsearch
>>> elasticsearch.VERSION
(5, 4, 0)
```

```
$ pip freeze | grep elasticsearch==
elasticsearch==6.3.1
```

**Changes**

Switch the order of elasticsearch2 and elasticsearch
in requirements.txt.

**I have no idea why this fixes it...**

**Side Effects**

**Ticket**

No ticket

h/t @binoculars and @mfraezz

HalcyonChimera/osf.io,Johnetordoff/osf.io,aaxelb/osf.io,HalcyonChimera/osf.io,brianjgeiger/osf.io,CenterForOpenScience/osf.io,HalcyonChimera/osf.io,brianjgeiger/osf.io,saradbowman/osf.io,brianjgeiger/osf.io,mfraezz/osf.io,adlius/osf.io,mattclark/osf.io,felliott/osf.io,Johnetordoff/osf.io,pattisdr/osf.io,cslzchen/osf.io,aaxelb/osf.io,aaxelb/osf.io,aaxelb/osf.io,mfraezz/osf.io,cslzchen/osf.io,felliott/osf.io,CenterForOpenScience/osf.io,CenterForOpenScience/osf.io,pattisdr/osf.io,HalcyonChimera/osf.io,cslzchen/osf.io,adlius/osf.io,felliott/osf.io,baylee-d/osf.io,adlius/osf.io,mattclark/osf.io,pattisdr/osf.io,brianjgeiger/osf.io,saradbowman/osf.io,Johnetordoff/osf.io,Johnetordoff/osf.io,mattclark/osf.io,adlius/osf.io,felliott/osf.io,mfraezz/osf.io,baylee-d/osf.io,mfraezz/osf.io,cslzchen/osf.io,CenterForOpenScience/osf.io,baylee-d/osf.io