Fix warnings for size and expand_scale from ggplot2 0.3.3/0.3.4

[jthomasmock]

• Replace ggplot2::expand_scale() with ggplot2::expansion() in vignettes to avoid deprecation warnings in ggplot2 0.3.3
• Replace size argument in line functions (geomsv.R) with linewidth to avoid deprecation warnings in ggplot2 0.3.4

Closes #78

No warnings/notes/errors on local testing