Hello, I recently set a pygmt environment on a new machine. I used anaconda and the version 0.3.0 package available on condaforge (conda install -c conda-forge pygmt=0.3.0).

I was running a script that uses the transparency option ( `fig.grdimage(imageFile.data,C="shakemap_pgv_mks.cpt",t=60))`

and I got the following warning:

`psconvert [WARNING]: Input file has transparency but your gs version 9.52 has a bug preventing it - please downgrade to 9.50`

And sure enough the transparency option was ignored and the plot was generated without the required transparency.

My question is how can I downgrade gmt (and thus psconvert) to an older version that supports the transparency option?

I tried conda install -c conda-forge gmt=6.1.1 but I still get the same error.

This will reproduce the error (if you create an fresh anaconda enviroment using the current version 0.3.0 of gmt in anaconda forge).

```
import pygmt
import pandas as pd
import xarray as xr
import numpy as np
### constants
sw4origin = (39.3058,-120.0052)
#sw4Xlength,sw4Ylength = 34000,34000 #in meters
#figure out the end of my region based on these dimenstions
#sw4OriginUTM = utm.from_latlon(sw4origin[0],sw4origin[1],11,"N")
#sw4EndX,sw4EndY = sw4OriginUTM[1]+sw4Xlength,sw4OriginUTM[0]+sw4Ylength
sw4EndLat,sw4EndLong = (39.62135044626656, -119.62277186293804)
region = [-120.0052,sw4EndLong,39.3058,sw4EndLat]
lat0,lon0=(39.310047, -119.999954)
latend,lonend = (39.610416, -119.61958)
### generate 2 fake xarrays
sampleXarray = np.zeros(shape=(128,145),dtype=np.float32)
for i in range(sampleXarray.shape[0]):
for j in range(sampleXarray.shape[1]):
sampleXarray[i,j] = i+j
underlayedXarray = np.copy(sampleXarray)
sampleXarray = np.abs(np.sin(sampleXarray))
#plt.imshow(sampleXarray)
#correctly georeference these fake xarray
xcoords = np.arange(lat0,latend,(latend-lat0)/sampleXarray.shape[0])
ycoords = np.arange(lon0,lonend,(lonend-lon0)/sampleXarray.shape[1])
dims = ['x','y']
coords = {"x":xcoords,"y":ycoords}
sampleXarray = xr.DataArray(sampleXarray,dims=dims,coords=coords)
underlayedXarray = xr.DataArray(underlayedXarray,dims=dims,coords=coords)
#attempt to plot, show that transparency is ignored
fig = pygmt.Figure()
fig.basemap(region=region,projection="M8i", frame=True)
#plot the underlying xarray (in my real problem this would be my hillshade)
fig.grdimage(underlayedXarray,C="gray")
pygmt.makecpt(cmap="viridis", series=[0.0, np.max(sampleXarray.data)], continuous=True)
fig.grdimage(sampleXarray, C=True, t=60)
fig.show()
```

Thank You,