Add package version numbers to imports

[mariabnd]

Detailed description

Add current version numbers of packages used in the imports of DESCRIPTION to hopefully avoid the worst potential future dependency issues

Context

• devtools 1.13.6
• eurostat 3.2.2
• dplyr 0.76
• stringr 1.3.1
• lubridate 1.7.4
• XML 3.98-1.16
• ggplot2 3.0.0
• here 0.1

Possible implementation

e.g.

Imports: 
  here (>= 0.1)